Evolutionary convergence and parallelism in crinoid calyx design

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Abstract
Eleven basic constructional designs are defined among the Crinoidea. Calyx designs are defined primarily by shape, geometry of plates at the base of the calyx, tegmen shape, fixed calyx plates, number of arms, and nature of radial facets. These calyx designs include the following: multiplated bowls, multiplated bicones, conical mosaics, urns, cylinders, cones, ellipsoids, hands, bowls, bilateral recumbents, and fists. Most crinoid subclasses and orders contain crinoids with several of these designs. This significant convergence and parallelism in calyx design indicates that the morphological evolution of crinoids has been substantially constrained by constructional and fabricational limitations.